1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints images by using an elongate print head comprising a plurality of printing element substrates, each having arrays of a plurality of ink ejection printing elements. More particularly it relates to a printing method to prevent seam stripes and density unevenness caused by print areas of the plurality of printing element substrates overlapping one another. It also relates to a printing method to prevent seam stripes and density unevenness that would otherwise occur at boundaries between print areas of a plurality of print scans.
2. Description of the Related Art
Ink jet printing apparatus in general are classified largely into two types—serial type and full line type. The serial type ink jet printing apparatus forms an image by repetitively alternating a printing main scan that moves the print head in a main scan direction while ejecting ink and a sub-scan that conveyes a print medium a distance corresponding to a printing width in a direction crossing the main scan direction.
The full line type ink jet printing apparatus on the other hand uses an elongate print head the length of which corresponds to a print width of a print medium. An image is formed on the print medium by ejecting ink from the individual printing elements of the print head at a predetermined frequency while at the same time conveying the print medium in a direction crossing the direction of arrays of the printing elements at a speed corresponding to the predetermined frequency. Such a full line type printing apparatus can output an image faster than the serial type.
Since an elongate print head is constructed of a large number of ink droplet ejection printing elements arranged in line at high density, it is extremely difficult to ensure that not a single printing element will result in an ejection failure, unavoidably leading to a reduced yield in a manufacturing process. Under this circumstance, it is generally considered effective in recent years to realize an elongate print head by fabricating a number of printing element substrates which have a relative small number of printing elements and joining them together. An elongate print head of this construction is referred to as a “joining head” in this specification. The joining head, while it can be effectively used on the full line type printing apparatus, can also be applied to the serial type printing apparatus. With the joining head, the serial type printing apparatus can complete an image with fewer printing main scans, improving the printing speed.
Such a “joining head” has a problem that when a plurality of printing element substrates, each having a plurality of ejection ports arrayed therein at a predetermined pitch, are put in place, it is unavoidable that some placement errors occur among the substrates. Such substrate placement errors translate into inclinations among individual printing element substrates and variations in distance between the printing element substrates and a print medium, causing image impairments, such as black stripes and white stripes, to appear at merged portions of images formed by different scans.
To deal with this problem, Japanese Patent Laid-Open No. 2003-305853 for example discloses a method for arranging joints of printing element substrates with high precision and a method for reducing deviations in alignment pitch of ejection ports by an alignment device to physically improve manufacturing precision.
Japanese Patent Laid-Open No. H05-57965 discloses a construction in which, as shown in FIG. 13, jointing portions of printing element substrates C41-C46 (chip joints) are arranged to have an overlapping portion D made up of a predetermined number of ejection openings or ports (printing elements). With this arrangement, arrays of dots printed by the ejection ports of the overlapping portions D of two substrates are interpolated by those dots ejected from two kinds of ejection ports of other substrates, thereby distributing dot position errors, that are likely to concentrate on the jointing portion, over the entire overlapping portion.
Another method has been proposed which involves differentiating ink volumes ejected from ejection ports at the jointing portion from ink volumes of other portions, in order to make unwanted stripes, that are likely to appear at the jointing portion, less noticeable. Further, in a construction that uses a plurality of ink colors, a method has been conceived which differentiates the positions of the overlapping portions among different colors so that positional errors of the jointing portion of different colors show at different positions.
In addition to the black or white stripes caused by the placement errors of the plurality of printing element substrates, the joining head has another problem of a density unevenness that is caused by a difference in ink ejection timing between the overlapping portions and other portions.
The problem of density unevenness will be explained in detail by referring to FIG. 13. An area A on a print medium which is printed by other than the overlapping portion D of the print head takes, for completion of its printing, a length of time in which it passes a width w of one of the printing element substrates C41-C46. On the other hand, printing an area B on the print medium, which is printed by the overlapping portion D of the print head, takes a length of time in which it passes a width w′ equivalent to two of the printing element substrates C41-C46. That is, the printing time of the area B is more than two times that of the area A.
In an image printed by the ink jet printing system in general, even if the same volumes of ink are used, a printed image density tends to be higher in an area applied with ink in a longer period of time than in an area applied with ink in a shorter period of time. In the example of FIG. 13, therefore, even if a uniform image is printed on a print medium P, a relatively low density area A and a relatively high density area B appear alternately, causing density unevenness. Such density unevenness does not pose any problem when only a black ink, for example, is used in producing a text image. However, when producing photographic images equivalent to silver salt pictures using such inks as light cyan and light magenta, in addition to basic four colors—cyan, magenta, yellow and black—as is increasingly performed in recent years, the density unevenness described above constitutes serious image impairments.
However, the conventional measures described above are taken to deal with black and white stripes caused by placement errors of the plurality of printing element substrates but not to deal with density variations caused by differences in printing time between overlapping portions and other portions. That is, in producing photographic images with such a high level of resolution as required in recent years at high speed by using an elongate “joining head”, it is still not possible to output satisfactory images.
The similar problem may also be observed in a serial type printing apparatus that does not use the “joining head”. As already explained, the serial type printing apparatus forms an image by alternating the printing main scan, which moves the print head as it ejects ink, and the sub-scan, which conveys the print medium a distance corresponding to the printing width in a direction crossing the printing main scan. Therefore, at boundaries between bands printed by individual main scans, images are printed by two main scans. As a result, these boundaries have increased densities, showing up as seam stripes.